You start out with Charm of Might and an augmentation for it. Note that it has a damage shield clicky-effect. If you lose your Charm or Augmentation, Gene and Walter can get you a new one.
Find Nacho for free low-level clarity and healing potions.
Once you earned some coins, you can buy basic armor from Fluffy and Roughy.
You can move to places the normal way, e.g. using the stones in the Planes of Knowledge. Or you can talk to one of the dungeon masters, who can send your party straight to a dungeon. They will send all of your party there, who are in range of 150. There are also other NPCs nearby that can teleport you to locations.
Initially you can level up to 52. You have access to zones of all eras, with the standard level restrictions.
To unlock level 55, you need to complete either the group or solo Tier 1 progression instances. For group, defeat Lady Vox and Lord Nagafen in an instanced zone; for solo defeat GrimClaw and Allizewsaur.
For group instances, you need to gather a group preferably with a Might of not more than 6. You can check your might with the Server Greeter. The suggested Might for each instance can be found in the Might-Table.
Then go to Permafrost or Nagafen's Lair, and hail the NPC at the zone-in. (Marta in Permafrost, Linda in Nagafen's Lair.) Request the instance, then hail again, and enter.
Upon defeating a progression NPC, its spirit will spawn. Hail the spirit with each of your characters to get the flag.
Once you have gathered all the flags of an era, you unlock the instances of the next era. You can check your current status with Sir Daniel at the hub.
After completing an era, remove your augments from your charm and hand it to Charlotte. She will return you an upgraded version.
The absolute difficulty of an instance is always the same. It is the Might of your party at the time of requesting the instance which defines the instance mode (i.e. the relative difficulty).
Normal is the default mode when requesting an instance. Defeating the boss in a normal instance yields a chest with additional loot, such as Tokens of Might.
Easy means you exceed the suggested Might. There is a lesser reward at the end.
Hard mode is automatically enabled if you bring half or less the suggested Might. Hard instances spawn an additional chest - see also Cloe. The first hard mode that should be doable for new players is Innoruuk.
Trivial is when you bring an excessive amount of Might to an instance. There is no chest reward in this mode.
The NPC spirit required for progression spawns in all modes. It does not spawn when killing the boss in the open world.
All instances up to and including Kunark are designed for a party size of 6. You can reach the maximum allowed Might for normal mode with 6 characters. Experience in a group of 6 is about 5-10% better than in a raid of 6.
Starting in Velious, the instances allow more Might than a progression group of six can fill. The Velious progression path A can be completed with 6. Path B is a bit tough for 6, and particularly NToV is something you may want to do with a partner and choose the easy mode.
In Luclin you probably want to either team up with someone or play 9. Seru and Emperor can be done with 6, but Vex Thal will be rather slow and challenging for 6.
Even though not impossible to beat with 6, Dragons and Idols is designed for two people playing 6 each or someone playing 9.
Soloing, 2-Boxing and 3-Boxing, it is possible to beat some of the group progression targets, but it is considerably harder than with a full group.
Keep in mind, the concept of Might considers the potential of a level. A level 54 and a level 55 with the exact same gear and AA don't differ much. However, the potential of level 55 is quite a lot higher than that of a level 54, because at 55 you gain access to new AAs and gear with required level of 55 (e.g. Seru/Emp). That is why there is a bit of a jump in Might for certain levels, such as from 59 to 60.
If you want to attempt Lady Vox in hard mode with newish characters (which btw. is really hard!), your chances with four characters of level 52 (total Might 4) are much better than with one of 56.
One idea behind the concept of Might is to have useful alt characters or lower level characters. You can use the lower level characters on their own, or you can add them to extend a higher level team. Velious allows 75 Might. That is for example 7 level 60 (70 Might) and 2 level 53 (4 Might).